Yesterday, at PubCon, Gary Illyes from Google spoke. I was not there but I followed much of the coverage of it on Twitter. One thing he said that I saw from multiple tweets was that when you do block thin or bad content, Google prefers when you use the noindex over 404ing the page.
Jenny Stradling posted this on Twitter:
When you use the noindex, Google probably will remove the content faster versus 404ing it which means Google has to detect there is a 404 status code. Also, many webmasters mess up on 404ing their content, using soft 404s instead.
Of course, Gary recently said that it is best not to remove content but make content better when it comes to Panda.
Forum discussion at Twitter.
Update: Jennifer Slegg has a more context on what Gary said over here.